This invention relates to water jet propulsion systems for marine vehicles and in particular to means for making these propulsion systems more efficient. Water jet propulsion systems for marine vehicles usually comprise one or more pumps which pump water in through one or more inlets mounted on the bottom of the vehicle and discharge it out through nozzles which are pointed in a direction which is opposite from the direction of travel of the vehicle. The amount of thrust which is generated by such a system is directly proportional to the rate at which the system adds momentum to the water that is pumped in through the inlet and out the nozzle. The amount of momentum which the propulsion system will add to this water during any fixed period of time will be directly proportional to the volume of water which is pumped through the system, and to the difference between the momentum velocity of water entering the inlet and the velocity of the water leaving the nozzles. The momentum velocity of the water entering the inlet represents the velocity of the inlet entering water relative to the hull in the region from which the water is obtained. This relationship is shown by the equation EQU T = .sigma. Q(V.sub.j -V.sub.m)
where T equals the thrust, .sigma. equals the density of water, Q equals the flow rate of water in volume per unit time, V.sub.j is the jet velocity of the water leaving the nozzle, and V.sub.m is the momentum velocity of the water entering the inlet.
The amount of thrust that is produced by any actual water jet propulsion system will always be less than the value obtained from the above equation because this equation does not account for the various kinds of losses which will occur in any actual propulsion system. If the water velocity through the system is too high, low pressure points may be created at various places in the system which will cause the water to cavitate. This cavitation will result in large numbers of bubbles which will seriously restrict the flow rate of water through the propulsion system and thus lower the thrust. The bubbles cause by cavitation may also cause erosion and eventually damage to various parts of the system. Kinetic energy is also wasted because of friction between the high velocity water flow through the ducts, pumps and nozzles of the system. The losses in thrust for this reason can be reduced by minimizing the velocity of the water through the system and by minimizing the length of the nozzles and water ducts. The nozzles at the outputs of the pumps are made very short because the jet velocity of the water passing through the nozzles is much higher than the velocity of the water in any other part of the propulsion system.
As can be seen from the above equation, if the water jet velocity V.sub.j is reduced and the thrust produced by the propulsion is to remain constant, then either the flow rate Q must be increased or the water inlet velocity V.sub.m must be decreased. The trend of development in prior propulsion systems has been to increase efficiency by decreasing the water jet velocity V.sub.j and to compensate by increasing the flow rate Q so as to maintain the same amount of thrust. The velocity of water through any part of the propulsion system will always be inversely proportional to the cross sectional area of that part of the system and directly proportional to the flow rate through that part of the system. Therefore, when the prior art systems were made more efficient by decreasing the water jet velocity V.sub.j and increasing the flow rate Q, it was necessary to compensate by increasing the size of the water pumps and the nozzles. It was also necessary to increase the size of the water inlets if an increase in the water inlet and ducting velocities were to be avoided. Thus each successive increase in system efficiency resulting from lowering the water jet velocity and increasing the flow rate forces the propulsion system to become larger and heavier.
One way to increase the efficiency of the propulsion system without at the same time decreasing the thrust would be to decrease the water inlet velocity V.sub.m. However, the minimum value of water inlet velocity that can be obtained is determined by the speed at which the marine vehicle is moving. The relative velocity of the water in the boundary layer very close to the hull of the marine vehicle, will always be much less than the velocity of the vehicle itself. The boundary layer water further away from the hull of the marine vehicle will be moving at a faster velocity with respect to the vehicle, and the relative velocity between the marine vehicle and any water beyond the boundary layer will be the same as the velocity of the marine vehicle through the main body of water. Thus, while the vehicle is moving, the water will always be forced into the inlet at some minimum velocity which is determined by the speed of the marine vehicle, the characteristics of the boundary layer surrounding the vehicle hull, and the shape of the water inlets. The prior art water inlets have been constructed so that the width dimension, measured along a line perpendicular to the length of the hull, has been from one to two but never higher than five times the height dimension of the inlets. In order to take enough water through these prior art inlets to satisfy the flow rate requirements of the propulsion systems, it has been necessary for the inlets to take in not only water from the boundary layer close to the vehicle hull but also water from outside the boundary layer which is moving at high velocities with respect to the hull. The resulting water inlet velocities of these prior art systems would therefore approach the value of the relative velocity between the vehicle and the main body of water.
Most of the prior art water jet propulsion system units comprise only one pump which is driven by only one motor. Operation of such a propulsion system at part power can be accomplished only by operating the motor at part power. Those prior art propulsion systems which have multiple pumps with each pump being connected to a separate motor can be operated at part power by shutting down one or more complete motors and pumps while operating one or more of the other motors and pumps at full up to power. When either of these methods of part power operation is used, the propulsion system will be operating much less efficiently than it is when operating at full power. Many kinds of power sources, such as gas turbines, will operate efficiently only when they are delivering close to their full power output. When these types of power sources are used in propulsion systems and operated at part power, the overall efficiency of the propulsion systems is greatly decreased. The kinetic energy loss which results when the high velocity water is emitted from the nozzles is porportional to the square of the water jet velocity. Therefore, in any propulsion system with two or more pumps, this loss will be minimized, for any fixed overall thrust, when the pumps are operated so that the water velocities through each of the nozzles are equal and therefore minimum. Thus when a water jet propulsion system with multiple pumps is operated at part power, this loss will be minimized when all of the pumps are operated at the same time instead of having one or more pumps turned off completely while the others are operating at high speed and therefore with higher jet velocities.